1. Field of the Invention
The present invention relates to a communication system and a resource assigning method therefor, and more specifically, to a communication system for obtaining a resource for communication when a request therefor is issued and releasing the resource when the communication is over so that a plurality of communications can share the same resource and that the amount of resources required varies among the communications, as well as a resource assigning method that enables resources to be more efficiently shared in this communication system.
2. Description of the Related Art
Communication systems require various resources for communication; communication systems in which a large number of users communicate with one another require communication circuits in exchanges, relay cables, and a specified amount of radio circuits, and mobile communication systems in which users communicate with base stations while travelling require radio channels for use in communication between mobile and base stations. For each of these communication systems, system design must provide an amount of resources commensurate with the number of users.
Typically, since the probability that all the large number of users simultaneously make communication is assumed to be very low and negligible, it is unnecessary to provide sufficient resources to allow all these users to simultaneously make communication. If, however, requests for communication require an amount of resources exceeding a predetermined value, the system cannot accept these requests, that is, a call loss occurs. The system design typically determines the amount of resources so that the probability (call loss rate) that a call loss occurs is low and about 1 to several percents. Such system design is described, for example, in L. Kleinrock, “Queuing systems Volume I: Theory,” John Willey & Sons, 1975 in detail, and description thereof is omitted.
Communications, which focused on sounds, have recently been diversified, and various aspects such as animated-image and data communications are emerging. Further, requests for communications have been diversified, and the amount of resources requested for communication is not always fixed; a varying amount of resources are requested. However, although individual small resources are constantly released and become free, they are unlikely to be combined into a large free resource. Accordingly, it is difficult to obtain free resources as required. Consequently, communication requiring a large amount of resources is likely to result in a call loss.
To solve these problems to maintain an impartial call loss rate, a method of providing a fixed amount of resources beforehand or the like is used. For example, Japanese Patent Application Publication No. 11-41239 “Method for Controlling Call Acceptances in Multidimensional Traffic” describes in detail an approach to obtaining an impartial call loss rate by using a simple method of avoiding accepting new resource requests if the number of resources in use exceeds a fixed threshold value.
Such system design is used for rapidly advancing mobile communication systems. In the mobile communication systems, a radio channel is used for communication between a mobile station and a base station. Typically, since a mobile communication system can use only limited radio bands, the amount of resources must be more strictly designed.
Radio access methods used for the mobile communication system include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA). In the FDMA and TDMA methods, the current traffic can be determined by counting the number of radio channels or time slots in use, and the design approach employed for fixed telephone networks as described previously is applicable to these methods.
On the other hand, with the CDMA method, the number of radio channels cannot be counted, but as described in WO98/30057 in detail, the traffic associated with a base station can be determined by measuring interference power at the base station or the total transmitted power therefrom. The conventional design approach or the multidimensional approach is applicable to the CDMA method.
In this manner, various system design approaches considering even multidimensional traffic or various traffic control approaches are conventionally applied to the various communication systems.
The conventional traffic control approaches, however, cannot avoid reducing the efficiency of resource sharing under multidimensional traffic conditions. In general, if the call loss rate is assumed to be fixed, the utilization of resources decreases with an increase in the amount of resources used for a single communication. This phenomenon is based on what is called the aggregation effect; the utilization of the resources increases consistently with the amount of available resources regardless of the amount of resources used for a single communication.
Thus, with the conventional design methods for multidimensional traffic or the traffic control approach disclosed in Japanese Patent Application Publication No. 11-41239, an impartial call loss rate is obtained, but the general utilization of the resources disadvantageously decreases if a larger number of communications each require a large amount of resources.